To establish a VPN connection from the VPN clients connected to the G34/G36, VPN traffic must be allowed to pass to VPN clients in the home network.  PPTP VPN is supported in the G34/G36.  By default, VPN passthrough settings are enabled.  One of many steps of troubleshooting for VPN connectivity issue is to make sure the VPN passthrough setting is enabled. The below document describes how to enable PPTP passthrough in the G34/G36.
http://arris.force.com/consumers/articles/General_FAQs/G34-G36-VPN-Passthrough-Setup